Surface emitting second harmonic generating device

ABSTRACT

A surface emitting second harmonic generating device capable of generating a second harmonic at room temperatures with high efficiency and output power, and which has a small size, low energy consumption and a low manufacturing cost. A second harmonic is efficiently generated when the &lt;100&gt; direction of the semiconductor crystals within a cavity makes an angle of 5° or more with respect to the direction of the light rays (particularly when one of the &lt;111&gt;, &lt;211&gt; and &lt;110&gt; directions approximately matches the direction of the light rays). Further, if a superlattice second harmonic generating layer composed of a III-V or II-VI compound semiconductor is provided between the output end reflector and the spacer layer, the second harmonic may be generated with even greater efficiency. In addition, the spacer layer may be formed by a superlattice, as may the active layer and the spacer layers. A second harmonic may also be efficiently generated by utilizing the spacer layer and the active layer as phase-matching layers.

FIELD OF THE INVENTION

The present invention relates generally to a surface emitting secondharmonic generating device, and more particularly to a surface emittingsecond harmonic generating device which is capable of highly efficientextraction of a second harmonic (especially monochromatic light, such asviolet, blue and green), in a direction perpendicular to a substrate.

BACKGROUND OF THE INVENTION

Lasers and light emitting diodes (LEDs) are used as sources of bluelight in various fields of optoelectronics, such as optical measurement,optical transmission and optical displays. Light-emitting devices whichuse LEDs (particularly those which emit blue light) utilizing GaNsemiconductors are known (see, e.g., refer to S. Nakamura, T. Mukai andM. Senoh: Jpn. J. Appl. Phys., Vol. 30 (1991) L1998). However, since theline width of LED light is wide (a single wavelength cannot be created),lasers have in recent years been more widely used than LEDs in the fieldof optoelectronics.

For example, with some ZnCdSSe semiconductor lasers, an acceptable bluelight output is obtainable (see, e.g., M. A. Hasse, J. Qiu, J. M.DePuydt and H. Cheng: Appl. Phys. Lett., Vol. 59 (1991) 1272).Nevertheless, under the present circumstances such devices can only beused upon cooling to extremely low temperatures, and therefore apractical light output cannot be obtained at a room temperature.

In addition, light-emitting devices that introduce high-power solidlaser light or high-power semiconductor laser light into non-linearoptical crystals (e.g., dielectric substances such as LiNbO₃ and KNbO₃,semiconductors such as GaAs, organic substances, etc.) to generate asecond harmonic are known (e.g., refer to A. Yariv: Introduction toOptical Electronics, 4th ed.; Saunders College Publishing, (Holt,Rinehart and Winston, 1991)). As shown in FIG. 6, in this type of devicea laser source 23 and non-linear optical crystals 24 are arrangedbetween a pair of optical reflectors 21, 22, and laser light is launchedthrough the non-linear optical crystals 24 to generate a secondharmonic, and blue light is extracted from the reflector 22 which hasthe higher transmission of the second harmonic. However, the larger thesize of the device, the greater the cost of its production, and since itis composed of a combination of multiple components, its problemsinclude extremely difficult control and unstable output.

Further, devices are known which extract a second harmonic from the endsurface of normally striped GaAs or AlGaAs semiconductor lasers (see,e.g., N. Ogasawara, R. Ito, H. Rokukawa and W. Katsurashima: Jpn. J.Appl. Phys., Vol. 26 (1987) 1386), but the power of the fundamental waveinside these devices is low due to a low end facet reflectivity. Also,the absorption loss is large due to the long cavity. These device haveeven greater difficulty in achieving a structure for phase matching.These disadvantages make it impossible to generate the second harmonicwith high efficiency.

Further thought has been directed towards extraction of a secondharmonic in the direction perpendicular to the cavity. See, e.g., D.Vakhshoori, R. J. Fisher, M. Hong, D. L. Sivco, G. J. Zydzik, G. N. S.Chu and A. Y. Cho: Appl. Phys. Lett., Vol. 59 (1991) 896. However, in adevice of the type disclosed in this publication, the output power ofthe second harmonic is small and the emitted light is distributed over awide range, therefore condensing of the light is difficult; thus, atpresent, a practical application for this device has not yet beenachieved.

SUMMARY OF THE INVENTION

The present invention was conceived with an object to overcome thedisadvantages described above, and to provide surface emitting secondharmonic generating devices capable of generating a second harmonic withhigh efficiency and at a high power at near-room temperatures (forexample, -30° C. to approximately +100° C.), and which also features asmall size, low energy consumption, and low manufacturing cost.

A surface emitting second harmonic generating device in accordance withthe present invention comprises, as a cavity, an active layer composedof semiconductor crystals of a III-V group compound; a first spacerlayer disposed on a first side of the active layer and a second spacerlayer disposed on a second side of the active layer; and a firstreflector disposed on the first spacer layer opposite the active layerand a second reflector disposed on the second spacer layer opposite theactive layer, wherein the second reflector transmits the second harmonicat a prescribed rate. According to the invention, the <100> direction ofthe semiconductor crystals makes an angle of at least 5° with respect tothe direction of light rays propagating in the cavity.

Preferably, one of the <111>, <211> and <110> directions of thesemiconductor crystals within the cavity approximately matches thedirection of the light rays.

In addition, in preferred embodiments of the present invention, aphase-matching layer of superlattice structure, composed of one of aIII-V and II-VI compound semiconductor and having an orientationsubstantially identical to that of the semiconductor crystals within thecavity, is disposed between the second reflector, which transmits thesecond harmonic at the prescribed rate, and the second spacer layer.

In addition, the second spacer layer may be composed of a superlatticewhich also functions as a phase-matching layer. Alternatively, theactive layer and at least one of the spacer layers may be composed ofsuperlattices which also function as phase-matching layers.

Other features of the present invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing an example of a second harmonic generatingdevice according to the present invention, providing a cross-sectionalview of the entire device.

FIG. 2 is a cross-sectional diagram showing each layer of the section IIin FIG. 1.

FIG. 3 is a cross-sectional diagram showing each layer of anotherexample of a second harmonic generating device according to the presentinvention.

FIGS. 4(A) and 4(B) are drawings showing matrices of non-linearcoefficients and substrate orientations for the purpose of explanationof the function of the present invention.

FIGS. 5(A), 5(B) and 5(C) are drawings showing matrices of non-linearcoefficients and substrate orientations for the purpose of explanationof the function of the present invention.

FIG. 6 is a drawing showing an embodiment of a second harmonicgenerating device according to the prior art.

FIG. 7 is a cross-sectional diagram showing an overview of a surfaceemitting laser.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Much attention has been given recently to vertical cavity surfaceemitting lasers (e.g., refer to "Surface Emitting Lasers" by Kenichi Igaand Fumio Koyama: Ohmusha, Ltd., 1990). These surface emitting lasersare used as, for example, (1) a light source for writing and readingdata in light magnetic disk devices, (2) a light source for medicalequipment which utilize photo-luminescence, and (3) a light source indisplays which utilize a laser.

As shown, for example, in FIG. 7, such types of surface emitting lasersare constructed in such a way as to include a vertical cavity 33containing an active region 32, which is constructed on a (100)substrate (i.e., a crystal base plate whose surface orientation matchesthe (100) crystallographic axis direction), and reflectors 34, 35 whichare formed on both ends of the vertical cavity and have a highreflectivity. The reflectors 34, 35 generally are semiconductormulti-layer films or dielectric multi-layer films. At room temperatures,such surface emitting lasers exhibit single mode lasing with a singlewavelength up to an output power of a few milliwatts. The lightintensity inside the cavity is estimated to be over 100 times the outputpower, or a few hundred milliwatts.

The inventors of the present invention noted the following properties ofa III-V compound semiconductor vertical cavity surface emitting lasersuch as the one shown in FIG. 7:

(1) the efficiency when converting a fundamental wave to a secondharmonic is in proportion to the power of the fundamental wave, and thusif the second harmonic can be generated inside the vertical cavitylaser, the light intensity of the fundamental wave is extremelyfavorable to generation of the second harmonic;

(2) the vertical cavity laser includes a pair of reflectors, one ofwhich may be characterized to transmit the second harmonic; and

(3) the crystals comprising it have a large non-linear coefficient.

Additionally, further research revealed that a phase-matching layerformed inside the vertical cavity surface emitting laser can contributeto manufacture a second harmonic generating device that is smaller, haslower energy consumption, and has a lower production cost.

A surface emitting second harmonic generating device according to thepresent invention is based on the above mentioned facts, and comprises,as a cavity, an active layer composed of semiconductor crystals of aIII-V compound; spacer layers constructed on each side of the activelayer; and a pair of reflectors constructed one on each spacer layer andpositioned on the side opposite from the active layer, where one of thereflectors transmits the second harmonic at a certain rate. The deviceis characterized in that the <100> direction of the semiconductorcrystals within the cavity makes an angle of 5 degrees or more with thedirection of the light rays. Preferably, one of the <111>, <211> and<110> directions of the semiconductor crystals within the cavity roughlymatches the direction of the light rays.

Here, <a b c> (a, b, c are integers) are symbols indicating directionsequivalent to [a b c]; for example, <100> signifies the directions shownin Table I.

                  TABLE I                                                         ______________________________________                                        [100]           [010]  [001]                                                  [- 100]         [0- 10]                                                                              [00- 1]                                                ______________________________________                                    

The principle behind second harmonic generation in the context of thepresent invention is summarized below. Overlined numbers are customarilyused to indicate crystallographic axes/directions and orientations, buthereinafter a minus symbol is added to the displayed numbers instead.

Generally, a non-linear optical crystal is used when generating a secondharmonic from a fundamental wave. In accordance with the presentinvention, a zincblende-type crystalline structure, such as GaAs, etc.,may be used as such a crystal. When light is propagated through azincblende-type crystal, a secondary (i.e., twice the fundamentalfrequency) non-linear polarization P_(NL) is induced in the crystal, anda second harmonic is generated which has an electrical field inproportion to the size of the polarization P_(NL).

Further, the amount of the generated second harmonic depends on theorientation of the semiconductor crystal of the cavity (including thephase-matching layer). Since this orientation is usually the same asthat of the crystal making up the substrate, the relationship betweenthe substrate orientation and the fundamental wave is actually therelationship between the crystal orientation making up the cavity andthe fundamental wave, and thus, hereunder, the substrate orientation istreated as the crystal orientation of the cavity.

A case will now be considered wherein the x-axis of an x-y-z orthogonalcoordinate system is set along the direction normal to the substratesurface and a fundamental wave is propagated in the direction of thex-axis. The x, y, and z components of the polarization P_(NL), i.e.,P_(NLx), P_(NLy), and P_(NLz) may be represented as shown in Equation 1:##EQU1##

Here, P_(NLx), P_(NLy) and P_(NLz) represent the x, y and z componentsof the polarization P_(NL), and the matrix elements dmn (m=1, 2, 3; n=1,2, . . . , 6) represent the coefficients relating the non-linearpolarization to the electric field of the fundamental wave. The electricfield components E_(x), E_(y), E_(z) are the x, y and z components ofthe light fundamental wave. Also, since the fundamental wave ispropagated along the direction of the x-axis, E_(x) can be set to zero.

Matrices which include the symbol dmn are often only defined for thecase where the x-axis is [100], the y-axis is [010] and the z-axis is[001]. Here, the definition is generalized in order to use the samesymbol for any arbitrary set of orthogonal axes x, y and z.

Assume a substrate such as the one shown in FIG. 4(A), wherein [100] isdirected to the x-axis, [010] to the y-axis, and [001] to the z-axis. Inthis substrate, as shown in FIG. 4(A), the matrix element dmn holdsnon-zero values for d₁₄ =d₂₅ =d₃₆ (in FIG. 4(A) this is set to 1.000 forconvenience of explanation), while the other elements of the matrix arezero. If fundamental light is propagated along the x-axis within thesubstrate, P_(NLy) and P_(NLz) both become zero, and therefore thegenerated second harmonic is not propagated along the x-axis (thedirection perpendicular to the substrate surface).

As shown in FIG. 4(B), when the substrate crystal in FIG. 4(A) arerotated -45° around the x-axis, that is, when [100] is directed to thex-axis, [011] to the y-axis, and [0-11] to the z-axis, the secondharmonic is not propagated along the x-axis, as is clear from the matrixelements.

For the case that the x-axis is not directed to [100], the change inefficiency of generating the second harmonic will be studied by using afew representative embodiments, wherein the substrate shown in FIG. 4(B)is rotated around the z-axis (or [0-11] axis) from the x direction tothe y direction. These embodiments are shown in FIGS. 5(A)-5(C). Theangles of rotation are -5°, about -35.3° and about -54.7°, respectively.In FIG. 5(B), the x-axis is parallel to [211], the y-axis to [-111] andthe z-axis to [0-11], while in FIG. 5(C) the x-axis is parallel to[111], the y-axis to [-211] and the z-axis to [0-11].

The non-linear coefficient matrix elements corresponding to thecoordinate axes are shown respectively in FIGS. 4(A) and 4(B), and inFIGS. 5(A)-5(C). As is clear from these matrix elements, if E_(x) =0 forthe rotation operation from FIG. 4(B) to FIG. 5(C), then the non-linearpolarization P_(NLy) and P_(NLz) which contributes to the secondharmonic propagated along the x-axis may be expressed as follows:

    P.sub.NLy =d.sub.22 E.sub.y E.sub.y +d.sub.23 E.sub.z E.sub.z

    P.sub.NLz =2d.sub.23 E.sub.y E.sub.z

    (Note that d.sub.34 =d.sub.23)

In simple terms, in this rotation, since -d₂₂ ≧d₂₃ ≧0, the polarizationparallel to the y-z plane

    P.sub.NL =(P.sub.NLy.sup.2 +P.sub.NLz.sup.2).sup.1/2

under the constant fundamental wave

    E=(E.sub.y.sup.2 +E.sub.z.sup.2).sup.1/2

has its maximum value when the fundamental wave is polarized along they-axis (that is, E_(z) =0, E=E_(y)).

On the other hand, if the fundamental wave is polarized along they-axis, as the angle of rotation is larger, P_(NL) increases and has amaximum value in the case of FIG. 5(B). When the angle of rotation isfurther increased, it decreases again, but since -d₂₂ =d₂₃ in the caseof FIG. 5(C),

    P.sub.NL =-d.sub.22 E.sup.2,

providing a very favorable property such that the efficiency of thesecond harmonic generation is independent of the polarization of thefundamental wave.

The actual calculation is not included here, but if the ratio is takenof the square of P_(NL) in the case of FIG. 5(A) and the square ofP_(NL) in the case of FIG. 5(C), then

    (0.260/0.816).sup.2 =0.1

The above equation supports that when the <100> direction of thesubstrate (i.e., the <100> direction of the semiconductor crystalswithin the cavity) makes a 5° angle with respect to direction of thelight rays, it is possible to obtain roughly 10% of the second harmonicgeneration obtained when the <111> direction of the substrate (i.e., the<111> direction of the semiconductor crystals within the cavity) matchesthe direction of the light rays. If such 10% efficiency can be achieved,then an adequate practical use thereof is apparent, and this explainswhy the <100> direction of the semiconductor crystals within the cavitymakes an angle of 5° or more with respect to the direction of the lightrays.

The particulars of a case where the rotation is in a direction differentfrom the one described above or where the rotation is around the y-axisare not described herein, but basically, in these cases as well, thefollowing statements are true:

(1) as the angle between the x-axis and the <100> direction increases,so does P_(NL) ;

(2) when the x-axis is in the direction <211> (see FIG. 5(B)) or in thedirection <110>, the efficiency is maximized if the polarization of thefundamental wave is appropriate;

(3) when the x-axis is in the direction <111>, an efficiency isachievable which is slightly less than that in (2) above, but it doesnot depend on the polarization of the fundamental wave.

Some prior studies disclose the substrate's <100> direction oriented atan angle of about 4° with respect to the direction of the light rays.However, this is based on the fact that such an angle is favorable tothe growth of the crystals, and is not based on the generation of asecond harmonic. Therefore, the devices disclosed in these studies arenot constructed so that the reflectors transmit the second harmonic.

In addition, since technology does not exist for generating the secondharmonic using a surface emitting laser, there has been no attempt totake a phase-match for second harmonic generation.

As described above, if a second harmonic generating device isconstructed such that the <100> direction of the semiconductor crystalswithin the cavity makes an angle of 5° or more with respect to thedirection of the light rays (i.e., the bearing of the substratesurface), or such that any one of the <100>, <211> or <100> bearings ofthe crystals within the cavity roughly matches the direction of thelight rays, it is possible to make a highly efficient extraction of thesecond harmonic.

Also, if material with a large non-linear coefficient, such as GaAs,etc., is used as a zincblende structure crystal, then an even highergeneration efficiency of the second harmonic achievable.

As regards the second harmonic, when a phase match is not taken insidethe cavity, the second harmonic generation is not very efficient, due tonegative phase interference, etc. In order to prevent this, the presentinvention provides a phase-matching layer of a III-V or II-VI compoundsemiconductor superlattice constructed between the reflector whichtransmits the second harmonic and the spacer layer which is positionedon the reflector side. A variety of methods may be utilized to createthe phase-matching layer according to the present invention, including amethod which modulates the value of the non-linear coefficient and amethod which inverts the sign, etc.

When phase matching is done by modulation of the non-linear coefficient,the superlattice structure is alternately laminated with differentcompositions, i.e., a III-V compound semiconductor (such as AlGaAs,InGaAs, AlGaInP, GaInAsP, etc.) and a II-VI group compound semiconductor(such as ZnCdSSe, ZnSSe, ZnCdS, etc.). In this manner, it is possible toreduce the negative phase interference of the second harmonic.

When creating a phase-matching layer using a II-VI compoundsemiconductor, it is possible from its high transmission of the secondharmonic to widen the thickness of the phase-matching layer to a degree(e.g., about 30 μm) allowable by the fabrication process and whichsupports the lasing of the fundamental wave in the surface emittinglaser.

It is further possible to form the spacer layer at the second harmonicoutput side, as well as the active layer and both of the spacer layers,using a superlattice of a III-V compound semiconductor functioning as aphase-matching layer. In such a case, if the number of laminations ofthe superlattice is increased, the efficiency of generation of thesecond harmonic is improved, but if the number is excessively increased,a stronger electrical resistance results and the generation of thefundamental wave itself is reduced.

The following is a more concrete description of the above principle.

Along with the fundamental wave (for example, wavelength λ₁ =870 nm)inside the cavity, the second harmonic thereof (in this case, blue lightof wavelength λ₂ =λ₁ /2) is generated. As the fundamental wave isreflected inside the cavity by a reflector with a high reflectivity forthe fundamental wave (for example, a reflectivity of 99% or over), thepower of the wave becomes higher. On the other hand, the second harmonicis emitted out of the device by the output terminal reflector whichtransmits the second harmonic (for example, with a transmissivity of 50%or over).

The power of the fundamental wave is rather high in a vertical cavitysurface emitting laser. The efficiency of generation of the secondharmonic is proportional to the power of the fundamental wave, and thusa high-power second harmonic is generated.

Also, when the second harmonic is not phase-matched inside the cavity,the generated light is not efficiently amplified. It is thereforefavorable to include a phase-matching layer inside the cavity, or tohave the spacer layer and the active layer composed of a superlatticefunctioning as a phase-matching layer. Thus, a surface emitting deviceprovided in accordance with the present invention generates a secondharmonic with high efficiency.

Further, a second harmonic generating device according to the presentinvention may be constructed by using various types of surface emittinglasers, such as:

(1) a buried structure surface emitting laser;

(2) a mesa-cap structure surface emitting laser;

(3) a DBR surface emitting laser;

(4) a multi-quantum well surface emitting laser;

(5) a single-quantum well surface emitting laser;

(6) a multi-barrier surface emitting laser;

(7) a strained surface emitting laser; or

(8) combinations of (1)-(7);

and by adjusting the substrate orientation, and forming a superlatticephase-matching layer, and a superlattice spacer layer or active layer,etc.

In addition, in a surface emitting second harmonic generating deviceaccording to the present invention, III-V compound semiconductors, suchas GaAs, AlGaAs, GaInAsP, AlGaInP, GaInAsP, etc., may be utilized,depending on the wavelength of the fundamental wave, while GaAs, GaAsP,etc., may be utilized as the substrate.

FIG. 1 is a cross-sectional diagram of an example of a surface emittingsecond harmonic generating device according to the present invention.The device in this example has a cavity, reflectors, etc., describedhereinafter, which are constructed on a <111> GaAs substrate 1 (asubstrate with a substrate orientation of <111>). A formed electrode 2is formed on the bottom of the substrate, and a reflector 3 composed ofmulti-layer film is provided on the upper surface of the formedelectrode 2. Further, as described above, the orientation of thesemiconductor crystals within the cavity is identical to the substrateorientation, and thus the orientation of the latter determines that ofthe former.

Formed on the upper surface of the reflector 3 are a spacer layer 4, anactive layer 5 and a spacer layer 6. Also, an insulation layer 11 isformed between the electrode 2 and the spacer layer 4 (except for thecenter section).

An electrode 7 is formed on the upper surface of the substrate 1, andthe substrate is etched to form a second harmonic output port 8 on theside of the electrode 7.

In the output port 8, a phase-matching layer 9 is formed on the upperside of the spacer layer 6, and a reflector 10 for output of the secondharmonic on the upper surface of the phase-matching layer 9 is pairedwith the reflector 3.

The cavity is composed of the above-mentioned reflectors 3, 10 and thelayers formed between them (the layers comprised of the space layer 4,the active layer 5, the spacer layer 6 and the phase-matching layer 9).

In this example, the phase-matching layer 9 does not lie between theelectrodes 2, 7 (in the section through which no current flows), asshown in FIG. 1. Therefore, disadvantages such as a decrease in thepower of the fundamental wave due to an increase in electric resistancedo not arise.

FIG. 2 is an enlarged diagram of the vertical cavity in FIG. 1. In FIG.2, a dielectric multi-layered film (composed of alternating layers ofSiO₂ and TiO₂) is used to form the reflector 10 at the output side, bypiling up about 10 pairs (or more, depending on the process used) of thealternating layers, whose thicknesses t may be expressed as

    t.sub.d1 =λ.sub.1 /[4n.sub.d1 (λ.sub.1)]; and

    t.sub.d2 =λ.sub.1 /[4n.sub.d2 (λ.sub.1)], respectively.

Here, n(λ) is the refractive index at the wavelength λ, while the λ₁ isthe wavelength of the fundamental wave. The subscript d1 denotes thefilm thickness and refractive index for SiO₂, while the subscript d2denotes the film thickness and refractive index for TiO₂.

The phase-matching layer 9 has a superlattice structure and is composedof two AlGaAs layers with different contents of A1, represented by s1and s2 (content of A1 for s1: 50-90%; content of A1 for s2: 10-50%).

Here, the thickness t_(s1), t_(s2) of each layer s1, s2 are determinedusing their respective reflective indices n_(s1) (λ) and n_(s2) (λ) inthe following equation:

Equation 2

    t.sub.s1 =[n.sub.s1 (λ.sub.2)/λ.sub.2 -2n.sub.s1 (λ.sub.1)/λ.sub.1 ].sup.-1 /2

    t.sub.s2 =[n.sub.s2 (λ.sub.2)/λ.sub.2 -2n.sub.s2 (λ.sub.1)/λ.sub.1 ].sup.-1 /2

The total thickness of the phase-matching layer 9 is a few times greaterthan the reciprocal of the average absorption constant, i.e., about 2-20μm.

In the above example, the substrate orientation was <111> (i.e., a <111>substrate was employed); however, the present invention is not limitedto this, as a second harmonic may be generated, albeit with a differencein efficiency, using a substrate of a III-V compound semiconductorwherein its orientation is greatly different from <100>; this means thatthe substrate orientation makes an angle of 5° or more with respect tothe <100> direction of the substrate crystal.

The active layer 5 may be composed of AlGaAs, etc., its compositionbeing determined according to the wavelength of the fundamental wave.

The composition of the reflectors 3, 10 is not necessarily restricted todielectric multi-layered films, and may be, for example, ofsemiconductor multi-layered films, metallic films, or a combination ofmetallic and dielectric films.

The reflector 10 on the output end needs only to have a reflectivityhigh enough to cause lasing of the fundamental wave, and atransmissivity high enough to extract the generated second harmonic. Thereflector 3 on the non-output end need only have a reflectivity highenough to cause lasing of the fundamental wave. A semiconductormulti-layered film reflector may be used because its electricalresistance is lower than that of the dielectric multi-layered film.

In the above example, the superlattice of the phase-matching layer 9 wascomposed of an AlGaAs compound semiconductor, but it may be composed ofany III-V compound semiconductor, such as InGaAs, AlGaInP, GaInAsP, etc.It may also be composed of a II-VI group compound semiconductor such asZnCdSSe, ZnSSe, ZnCdS, etc.

As described above, when the phase-matching layer 9 is formed utilizinga II-VI group compound semiconductor, the width of the phase-matchinglayer 9 may be increased to a degree which is allowed by the fabricationprocess and which will support the lasing of the fundamental wave of thesurface emitting laser.

Besides the above embodiments, any well-known method (see, for example,those shown in the reference by A. Yariv and the reference by D.Vahkshoori referred previously) may be employed to realizephase-matching. For example, the second harmonic may also be effectivelygenerated even where the thickness of layers corresponding to Equation 2takes the following values:

    t.sub.s1 =[n.sub.s1 (λ.sub.2)/λ.sub.2 ].sup.1 /2

    t.sub.s2 =[n.sub.s2 (λ.sub.2)/λ.sub.2 ].sup.-1 /2

This method is well known to realize phase-matching in standing waves.

Also, those described before utilize modulation in the value of thenon-linear coefficients, but phase-matching is also possible throughchanging signs of the non-linear coefficients.

In addition to the method of modulation of a non-linear coefficientdescribed in the above example in which the non-linear coefficientchanges in a rectangular shape and each layer takes half part of phaseshift (phase shift between second harmonic propagating and secondharmonic being generated), any other ways of modulation may be used--forexample, modulation with continuously changing non-linear coefficient oreach layer not taking half part of phase shift, etc., as long as thenegative phase interference can be effectively prevented.

Furthermore, due to the finite thickness of the whole phase-matchinglayers, the period of modulation of non-linear coefficient has a broadspectrum and accompanies a corresponding tolerance.

It is also possible to introduce a superlattice structure into thespacer layer or the active layer in order to fulfill the phase-matchingcondition, combining the function of these layers with that of thephase-matching layer (they may also be utilized as the phase-matchinglayer). In other words, by utilizing (i) the spacer layer, (ii) thespacer layer or the active layer on the second harmonic output side, and(iii) both the spacer layer and the active layer as the phase-matchinglayer, it is possible to reduce the thickness of the phase-matchinglayer or eliminate it.

FIG. 3 illustrates an embodiment employing both the spacer layer and theactive layer as the phase-matching layer. In this case, the thickness ofthe layer functioning as the phase-matching layer is adjusted to be afew times as great as the reciprocal of the absorption coefficient. Asexplained above, introduction of the superlattice being used as thephase-matching layer into the spacer layer or the active layer causes anincrease in the electrical resistance of the semiconductor laser,resulting in a trade-off with efficiency of the second harmonicgeneration.

Nevertheless, the following statements are true:

(1) a phase-matching layer such as the one shown in FIG. 1 eitherbecomes unnecessary or may be constructed more thinly;

(2) absorption loss of the fundamental wave is reduced due to (1);

(3) the modes are well unified, also due to (1); and

(4) the fabrication process is facilitated, also due to (1).

In this case, a superlattice composed of two types of AlGaAs withdifferent contents of A1 (i.e., an Al_(x) Ga_(1-x) As/Al_(y) Ga_(1-y)As--superlattice, x=y, etc.) is used for the spacer layers 4', 6', whilea superlattice composed of GaAs and AlGaAs (i.e., a GaAs/Al_(z)Ga_(-1-z) As--superlattice) is used for the active layer 5'. Further,when the active layer is AlGaAs, it is formed by a superlattice composedof two types of AlGaAs with different contents of A1.

The present invention produces the following effects:

(1) A second harmonic may be emitted from a surface emitting device withhigh efficiency. Additionally, a wavelength from violet to green may begenerated. Further, a wavelength from red to ultraviolet may begenerated by modifying the materials, the substrate orientation, and thecomposition of the reflectors.

(2) A second harmonic generating device having a small size, low energyconsumption and low manufacturing cost may be provided by utilizing theinherent advantages of the surface emitting laser.

Consequently, since a circular, single mode second harmonic may begenerated, it may be efficiently coupled into fibers, etc. In addition,multiple devices can be easily arrayed, and it is possible to emitdifferent monochrome wavelengths from each of the arrayed devices.

What is claimed is:
 1. A surface emitting second harmonic generatingdevice, comprising, as a cavity:an active layer composed ofsemiconductor crystals of a III-V group compound; a first spacer layerdisposed on a first side of said active layer and a second spacer layerdisposed on a second side of said active layer; and a first reflectordisposed on said first spacer layer opposite said active layer and asecond reflector disposed on said second spacer layer opposite saidactive layer, wherein said second reflector transmits the secondharmonic at a prescribed rate; wherein the <100> direction of saidsemiconductor crystals makes an angle of at least 5° with respect to thedirection of light rays propagating in said cavity.
 2. A surfaceemitting second harmonic generating device according to claim 1, whereinone of the <111>, <211> and <110> directions of the semiconductorcrystals within said cavity approximately matches the direction of thelight rays.
 3. A surface emitting second harmonic generating deviceaccording to claim 1, wherein a phase-matching layer of superlatticestructure, composed of one of a III-V and II-VI compound semiconductorand having an orientation substantially identical to that of thesemiconductor crystals within said cavity, is disposed between thesecond reflector which transmits the second harmonic at said prescribedrate and the second spacer layer.
 4. A surface emitting second harmonicgenerating device according to claim 2, wherein a phase-matching layerof superlattice structure, composed of one of a III-V and II-VI compoundsemiconductor and having an orientation substantially identical to thatof the semiconductor crystals within said cavity, is disposed betweenthe second reflector which transmits the second harmonic at saidprescribed rate and the second spacer layer.
 5. A surface emittingsecond harmonic generating device according to claim 1, wherein thesecond spacer layer is composed of a superlattice which also functionsas a phase-matching layer.
 6. A surface emitting second harmonicgenerating device according to claim 4, wherein the second spacer layeris composed of a superlattice which also functions as a phase-matchinglayer.
 7. A surface emitting second harmonic generating device accordingto claim 1, wherein the active layer and at least one of the spacerlayers are composed of superlattices which also function asphase-matching layers.
 8. A surface emitting second harmonic generatingdevice according to claim 2, wherein the active layer and at least oneof the spacer layers are composed of superlattices which also functionas phase-matching layers.
 9. A surface emitting second harmonicgenerating device according to claim 3, wherein at least one of thespacer layers is composed of superlattices which also function as aphase-matching layer.
 10. A surface emitting second harmonic generatingdevice according to claim 3, wherein the active layer and at least oneof the spacer layers are composed of superlattices which also functionas phase-matching layers.
 11. A surface emitting second harmonicgenerating device according to claim 4, wherein at least one of thespacer layers is composed of superlattices which also function as aphase-matching layer.
 12. A surface emitting second harmonic generatingdevice according to claim 4, wherein the active layer and at least oneof the spacer layers are composed of superlattices which also functionas phase-matching layers.
 13. A surface emitting second harmonicgenerating device, comprising:an active layer composed of semiconductorcrystals of a III-V group compound; a first spacer layer disposed belowsaid active layer on a first side of said active layer, and a secondspacer layer disposed above said active layer on a second side of saidactive layer; and a first reflector disposed below said first spacerlayer, opposite said active layer, and a second reflector disposed abovesaid second spacer layer, opposite said active layer, wherein saidsecond reflector transmits the second harmonic at a rate of at least50%; wherein the <100> direction of said semiconductor crystals makes anangle of at least 5° with respect to the direction of light rayspropagating in said cavity, and wherein one of the <111>, <211> and<110> directions of the semiconductor crystals within said cavityapproximately matches the direction of the light rays.
 14. A surfaceemitting second harmonic generating device according to claim 13,wherein a phase-matching layer of superlattice structure, composed ofone of a III-V and II-VI compound semiconductor and having anorientation substantially identical to that of the semiconductorcrystals within said cavity, is disposed between the second reflectorand the second spacer layer.
 15. A surface emitting second harmonicgenerating device according to claim 14, wherein the second spacer layeris composed of a superlattice which also functions as a phase-matchinglayer.
 16. A surface emitting second harmonic generating deviceaccording to claim 14, wherein the active layer and the spacer layersare composed of superlattices which also function as phase-matchinglayers.